I don't think Moose was talking about setting the current in the steppers; he was talking about the steps per millimeter using a screw that's not listed in Prusa's table. That's not hard to calculate anyway. The steppers have a native "pitch" of 200 steps per revolution and we have 16X "microstepping" enabled on the Printrboard, so we have 3200 (micro)steps per revolution. Given that information and the "pitch" of the screw in millimeters per revolution, it's easy enough to calculate the steps per millimeter.

Just FYI, the stepper controller data sheet is available online (and attached here), and it tells how the calculation of motor current works.

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I wasn't referring to Moose lead screws. I was just saying sorry that the calculator doesn't support his lead screws. I came across the calculator, while searching for the stepper motor formula for myself, and thought the calculator might be handy for other users on the forum. What I was asking if anyone might know what the constant number 8 represents in the formula I found for VREF.VREF = Current(A) x 8 * Sense_Resistor (.1ohm RS) * 0.90 This is the calculation for a 90% workload which is what we are shooting for. My stepper motors are an assortment you must check the model numbers and verify to attempt Stepper Tuning. You must verify the resistor used on your board. Mine is a R100 which is .1ohm your Vref calculation will be different if it's not .1ohm.

Mark

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Last edited by New Passion on 2017-Jul-Fri-10-Jul, edited 1 time in total.

If you look at the document that I attached to a previous post, you will see a schematic on page 3 where it shows the current that goes through the motor passing through a sense resistor Rs. Current X Resistance = Voltage, so the motor current flowing through Rs generates a Sense Voltage proportional to the current. (That is, the current at that moment through that winding of the motor; there are two coils and two sense resistors.)

The schematic shows the Sense Voltage being compared to a reference voltage, which is created by a DAC (Digital to Analog Converter) that uses Vref as its analog reference input and a digital word as its digital input. So the DAC in this case is able to create voltages for the comparator in steps equal to 1/8 of the reference voltage. (We get the 16 microsteps by including both forward and reverse directions of the current). I think that's where the factor of 8 shows up on page 7.

But all of that has absolutely nothing to do with steps per millimeter. It has to do with how much current you send to the motor. Note that a stepper motor has just about the same total current flowing through it (the two coils combined) all the time, even when stationary. If the current is too low, the stepper will not have enough power and it will miss steps. If the current is too high, the stepper motor will run too hot. Since Vref is difficult to access on the Printrboard, most users don't try to actually measure it, but instead empirically adjust the motor current to a happy medium where the motor isn't overheating and yet has enough power to execute the moves without missing steps.

EDIT: Just re-read your post. I doubt any practical stepper motor is going to work with a sense resistor of 100 ohms; that's far too high.

The larger the resistor, the SMALLER the current that will flow through the motor. 100 ohm resistors would deliver an impossibly small current, and even 10 ohm resistors will deliver extremely small currents. The resistors on the Printrboard are 0.11 ohms, so if you have 10 ohm resistors you'll be trying to run your motors on about 1% of the current delivered by the Printrboard motor controllers. It does look like your board has potentiometers for setting the motor current (but of course within limits).

HOWEVER I just looked at the documentation for your Melzi board and it seems they do use 0.11 ohm resistors just like the Printrboard. I can't read the writing on your resistors well enough to be sure of their value. The controller chips are not identical, but similar (A4988 vs A4982).